This invention relates to case hardening of steel, and particularly to apparatus and methods in which steel is case hardened by the application of a pulse which is generally less than 100 milliseconds.
In conventional equipment of this type, an inductive pulse of high frequency or a laser pulse is used. Use of such a pulse results in very fine grained steel with a hardness at least corresponding to the hardness which the base material would reach with a sudden quenching from temperatures above the A.sub.C3 point. For martensitic steels, such conventional quenching methods result in a high degree of brittleness so that the tools must be subjected to further thermal treatment such as annealing in order to achieve an optimum compromise between brittleness and hardness. The hardness in this conventional quenching method is thus always less than the hardness which the tool would have immediately after the quenching operation. However, the pulse treatment which is previously known results in a fine grained structure which does not require subsequent heat treatment because the brittleness has been considerably reduced.
However, conventional heat treatment by pulse application results in uniform hardness only if the material has been previously hardened and heat treated or annealed. Virgin steels which are subjected to such single pulses often yield in a wide distribution of hardness values. This lack of uniformity is shown in varying hardness values among units and in variations within a single profile as for example when the pulses are applied to the sawteeth of a tool. Of course, in production the base material cannot always be a prehardened steel.